There are many chemical processes where solid particulates, such as catalyst, and a hydrocarbon gas are contacted. Frequently, chemical reactions and physical phenomena occur for a predetermined period of time in a reaction zone, contained in, e.g., a moving or fixed bed reactor. Often, the gas/solid contacting is in a continuous or semi-continuous manner (hereinafter may be collectively referred to as “continuous”) instead of a batch operation. In such an instance, catalyst particles may be introduced and withdrawn from the reactor, which can be at a higher pressure than the source of the catalyst particles, such as a regenerator.
Hydrocarbon conversion units can include a reactor with one or more moving bed reaction zones used in conjunction with a regenerator. The reactor can include several reactor zones and can be structured in the form of a stack, or be split into sections. Typically, the regenerator with an atmosphere containing oxygen operates at a lower pressure than the reactor with an atmosphere containing hydrogen. Once the catalyst is transferred from the lower pressure to the higher pressure, a lift may be used to transfer the regenerated catalyst to the reactor. After the catalyst is spent, another lift can be utilized to transfer the catalyst from the reactor to the regenerator. Generally, the separation of the atmospheres of the reactor and regenerator is wanted to prevent undesirable side reactions.
Several devices can be used to transfer catalyst from a lower pressure zone to a higher pressure zone. One option may be a transfer vessel having double block-and-bleed ball valves to control the entry of regenerated catalyst into and out of the vessel. The catalyst entering the vessel can be purged with nitrogen to remove oxygen, and pressured with hydrogen up to the reactor pressure before transfer into the reactor. After catalyst exits the vessel, the vessel can be purged with nitrogen to remove the hydrogen before filling again with catalyst. Such a transfer vessel can separate the hydrogen atmosphere of the reactor from the oxygen atmosphere of the regenerator.
Another transfer vessel can be a valveless lock hopper, such as disclosed in U.S. Pat. No. 4,576,712 (Greenwood) or U.S. Pat. No. 4,872,969 (Sechrist), that can include three sections. Generally, catalyst is received in a top section where it is intermittently transferred to a middle section. The middle section can allow catalyst to flow in before being transferred to the bottom section. A standpipe's diameter may be sized in the middle section so that gas flowing upwards can stop catalyst flow, while allowing catalyst flow through another section of the pipe. This may be achieved by the alternate opening and closing of equalizing valves positioned on a pipe communicating with all three sections and in a parallel relationship with the catalyst flow. As an example, when the equalizing valve between the top and middle section is open, the equalizing valve between the middle and bottom section is closed so gas flowing up the lower standpipe will prevent catalyst flow from the middle zone through the lower standpipe, yet allow catalyst flow into the middle zone through the upper standpipe. Repeated cycling of the equalizing valves will allow a controlled flow of catalyst from the low pressure of the regenerator to the high pressure of the reactor.
Introducing catalyst particles into a high-pressure reactor from a regenerator can pose difficulties. Generally, it is beneficial to maintain a continuous flow of catalyst to and from the reactor and regenerator to facilitate conversion of a hydrocarbon stream and cap thermal cycling of the regenerator screen. Typically, a surge capacity is provided before and after the catalyst transfer vessel, which can pass catalyst from one pressure zone to another in batches. Before the transfer vessel, a vessel can be provided to receive a continuous flow of catalyst from the regeneration vessel, and after the transfer vessel, a vessel can be provided for permitting a continuous flow of catalyst to a lift communicating with a reaction vessel. Often, the cycling of the transfer vessel is controlled by the level of catalyst in that vessel. However, such a control can lead to intermittent catalyst flow that can have a negative effect on the operation of the reactor and/or regenerator.
Consequently, there is desire to control the cycling of a catalyst transfer vessel to ensure the consistent flow of catalyst to and from the reactor and regenerator.